Wind turbines use their rotors to convert wind energy into electrical power that is to be fed into a power network. For this purpose, the rotors have at least one rotor blade, whereby most wind turbines have a rotor with three rotor blades. A wind turbine consists essentially of a rotor with a hub and rotor blades as well as with a machine nacelle that accommodates the generator and often a gearbox. The nacelle is mounted rotatably onto a tower whose foundation provides the requisite stability. Depending on the direction of the wind, the nacelle is turned into the wind in such a way that a great deal of wind energy can be converted into electrical power in the generator due to the rotation of the rotor. The wind turbines are started up by control electronics when the wind speeds are greater than a start-up speed and thus hold out promise of producing energy, and the wind turbines are switched off again when the wind speeds are greater than a switch-off speed. In order for the rotor blades to be rotated, they are rotatably mounted in the hub and are rotated into the desired position by means of a pitch drive. The rotation of the rotor blades into or out of the wind is called “pitching”. Here, depending on the angle of attack, the rotor blades have a greater or lesser uplift that causes the rotor to rotate. A pitch regulation unit can also stop the rotor in that the rotor blades are rotated into a position that brakes the rotor until it comes to a standstill, even if there is wind. If the rotor is supposed to remain at a standstill, the angle of attack of the rotor blades is selected in such a way that the rotor blades do not receive any uplift from the wind.
Normally, the rotor blades are rotated on the basis of a comparison of a target signal for the desired rotor blade position to an actual signal that corresponds to the current rotor blade position. Due to this differential signal, the drive motor for the rotor blade position is actuated and, via the motor shaft, it exerts a drive torque onto the gearbox in order to change the rotor blade position. External torques of the rotor blade due to the attacking wind forces are superimposed upon said drive torque, resulting in an effective rotational drive torque. Due to turbulences and wind gusts, however, very high torques can act briefly upon the rotor blade drive mechanism. As a result, load peaks can occur during operation of a wind turbine that are far greater than would be derived on the basis of the drive torque of the drive motor used for the rotor blade adjustment, since considerable forces and stresses in the drive mechanism can arise due to the mass inertia of the drive mechanism and due to the friction in the gearbox. The loads of the drive mechanism used for the rotor blade adjustment can even be intensified by mechanical resonances in the components of the wind turbine as well as by play in the individual components. Systems are also known that derive the external torques from a mechanical deformation of the rotor blades. Here, however, for purposes of achieving better control, it is difficult to derive a direct relationship with the loads and vibrations in the rotor blade adjustment mechanism. Therefore, for purposes of improving the service life, the reliability and the availability of the wind turbine, it would be desirable if a rotor blade control were available with which such load peaks or any intensifying resonances could be avoided.